1. Field of the Invention
This invention relates to electrical logic circuits and, more particularly, to emitter coupled logic (ECL) circuits for performing an EXCLUSIVE OR-AND logic function.
2. Prior Art
In the electrical arts logic circuits are classified into the functions they perform, such as the basic AND circuit and the OR circuit. Another way of classifying logic circuits is the particular technology by which the circuits are implemented. One such technology noted for its speed of response is emitter coupled logic (ECL).
In the design of larger and more complex circuits performing logic functions certain combinations of basic circuits recur. One such combination is the EXCLUSIVE OR (XOR) logic circuit having its output terminal as one input terminal to an AND circuit. This permits the output signal of the XOR circuit to pass through the AND circuit at the proper signal on the other input terminal of the AND circuit. One purpose for this combination is that the AND circuit serves as a logic gate by which the output signal of the XOR circuit may be multiplexed in combination with other circuit outputs onto a common signal line.
The prior art approach to implement this circuit is simply to connect an XOR gate to an AND gate. Such a combination of circuits has a certain response time determined by the gate delay of, first, the XOR gate and then the AND gate. Power consumption, an important consideration for ECL, is set by the power requirements of two gates. Finally, from the standpoint of using the XOR-AND combination as part of an integrated circuit device, the two logic gate circuits occupy a certain amount of valuable space on the substrate of the integrated circuit device.